Plasma processing has long been employed to process substrates (e.g., wafer or flat panels or other substrates) to create electronic devices (e.g., integrated circuits or flat panel displays). In plasma processing, a substrate is disposed in a plasma processing chamber, which employs one or more electrodes to excite a source gas (which may be an etchant source gas or a deposition source gas) into a plasma for processing the substrate. The electrodes may be excited by one or more RF signals, which may be furnished by one or more RF generators, for example.
In some plasma processing systems, multiple RF signals, some of which may have the same or different RF frequencies, may be provided to one or more electrodes to generate plasma. In a typical capacitively-coupled plasma processing system, for example, one or more RF signals may be provided to the top electrode, the bottom electrode, or both in order to generate the desired plasma.
In some applications, one or more of the RF signals may be pulsed. For any given RF signal, RF pulsing involves alternately changing the RF signal between a high power set point and a low power set point. When an RF signal from an RF generator (e.g., RF_GEN1) is pulsed, the amount of RF power delivered by RF_GEN1 to the plasma load changes depending whether the RF signal is pulsed high or pulsed low. The changes in the RF power level delivered to the plasma load result in changes in the plasma impedance. For example, the plasma impedance may be at one level when RF generator RF_GEN1 is pulsed high, and at another level when RF generator GF_GEN1 is pulsed low.
If other RF generators have their frequencies tuned to maximize their power delivery based on the plasma impedance that exists during the high pulse of the RF signal from RF generator RF_GEN1, these RF frequencies will likely result in inefficient power delivery when the plasma impedance changes due to the fact that the RF power level delivered by RF generator RF_GEN1 has changed when the RF signal from RF_GEN1 pulses low, for example.
To further elaborate on the frequency tuning aspect, a modern RF generator may self-tune its RF frequency in order to more properly match the output impedance of that RF generator to the plasma load. As the term is employed herein, the independently pulsing (IP) RF signal refers to the RF signal that pulses independently of other RF signals. Such independently pulsing RF signal may pulse in response to a command from the tool host or another control circuit for example. A dependent RF signal is an RF signal that tunes or changes its RF frequency in order optimize its power delivery to the plasma load in response to the pulsing of the IP RF signal.
In the prior art, the dependent RF generator that provides the dependent RF signal may sweep through multiple frequencies during its frequency tuning process (such as in response to a plasma impedance change event caused by the pulsing of the IP RF signal). The dependent RF generator may monitor the forward power and reflected power during the frequency self-tuning process to determine the RF frequency that most efficiently delivers power to the plasma load as it sweeps through different frequencies.
In theory, the prior art self-tuning works adequately for certain applications. However, the RF signal pulsing frequency specified by modern processes is generally too fast (e.g., 10 KHz or faster) for self-tuning feature of dependent RF generators to keep up. This is because, in part, multiple samples are needed for frequency self-tuning, which requires the tuning/dependent RF generator to operate at impractically high frequencies in order to perform acceptable frequency tuning.
If the RF frequency of a dependent RF generator is not tuned quickly enough to adapt to the changing plasma impedance (such as the plasma impedance change following a high-to-low or low-to-high transition of the IP RF signal), power delivery by that dependent RF generators remains inefficient until the dependent RF signal has its frequency tuned. The longer the dependent RF generator takes to tune its frequency, the longer the time period during which power delivery by that dependent RF generator is non-optimal.
In view of the forgoing, there are desired improved methods and apparatus for ensuring that the RF frequencies of the dependent RF generators can quickly react to changes in the plasma impedance caused by IP RF signal pulsing.